It is generally desirable to maximize the amount of data which can be stored on an information storage disc. Data is generally stored on a disc on concentric circular tracks. Obviously, it is desirable to fit as many tracks as possible on the disc surface, which means that the track should be as narrow as practical and as close together as practical. However, it is also necessary to position a transducer over the middle of any desired track, and to move it there as rapidly as possible from another track each time the data is to be accessed on the disc. As the tracks become ever narrower and are spaced closer and closer together, it becomes increasingly difficult to rapidly and reliably position the transducer. It is the ability to rapidly and reliably position the transducer over the center of a desired or target track while the disc is rotating, which limits the access time of the recording disc, access time being an extremely significant factor in the useability of any disc drive.
With magnetic storage discs, typically an open loop positioning system with no feedback control is used to coarse-position either one or an array of read/write transducers over a desired cylinder/track. With a flexible magnetic storage disc, this typically has been the only positioning system for the transducer. With hard magnetic storage discs, open-loop coarse-positioning is generally followed by closed-loop positioning using a positioning system with feedback control to precisely position the transducer over the center of the desired track. The closed-loop positioning system generally locks onto and follows a magnetic pattern of some kind recorded on a portion of each track. Most of these magnetic servo schemes use a significant portion of the recording surface for a magnetic servo pattern. Obviously, this reduces the amount of recording surface available for recorded data.
Thus, an objective of this invention is to reduce the access time to a given target track without the use of a closed-loop servo system. More particularly, an objective is to provide a disc drive with low access times without sacrificing disc surface space to servo data storage.
It is another objective herein to provide a simply-implemented system which can reduce the time necessary to accurately position a transducer over the center of the target track.
In the past, it has been identified as highly desirable in finally positioning a transducer to reduce the settling or ring-out time of the stepper motor. This is the time necessary to slow down and finally stop a transducer in its movement from one track to another. In order to achieve a reduction from about 65 milliseconds access time to below 40 milliseconds, open loop control methods were initially used wherein a minimum ring-out time was achieved by correctly timing the step input command pulses to a stepper motor. This open loop approach, however, has limited effectiveness, because motor parameters will vary due to manufacturing tolerances. Therefore, with such an invariant algorithm, the step response will not provide a minimum ring-out sequence for every motor. However, utilizing the principles of closed loop methods to minimize the step response ring-out time requires an external sensor and an extensive amount of electronics to produce the necessary feedback signal. Therefore, the problem is to reduce the settling or ring-out time of the stepper motor driven actuator without significantly increasing the overhead in software or hardware incorporated in the disc drive.